Review articleSurface plasmon resonance induced charge transfer effect on the Ag-ZnSe-PATP system
Graphical abstract
A magnetron sputtering system was employed to deposit Ag and ZnSe on the PS template. SERS spectra of the Ag-ZnSe-PATP system were obtained with 514 and 633 nm excitation lasers. By discussing the CT contribution to SERS enhancement under the action of different excitation wavelengths, some basis for studying the SERS mechanism of wide-band-gap semiconductors is provided.
Introduction
Metal nanoparticles (NPs) can generate strong Raman signals when in contact with molecular compounds. This phenomenon is called surface-enhanced Raman scattering (SERS). With its high sensitivity and good selectivity, SERS has been widely used for ultratrace analysis and nondestructive tracking detection in various fields [1], [2], [3], [4], [5]. Commonly accepted explanations for SERS include the electromagnetic enhancement mechanism (EM) and chemical enhancement mechanism (CM). The EM comes from the inherent localized surface plasmon resonance (LSPR) of the metal itself [6], [7], [8], [9]. This phenomenon can significantly enhance the Raman scattering signal of related molecules. The CM is more complicated than the EM and includes resonance excitation, interaction and charge transfer (CT) between the molecule and the substrate [10]. When the incident light energy matches the energy required for CT, the resulting electronic transition enhances the CT contribution [11]. In a SERS system, the EM and CM usually coexist [12], [13].
The surface plasmon resonance (SPR) generated by metal NPs can greatly enhance electromagnetic radiation, cause CT, and enhance various photoelectric effects [14]. Therefore, metal NPs are widely used in ultrasensitive analysis at the molecular level. However, metal NPs make a greater contribution to electromagnetic field enhancement, interfering with the study of chemical mechanisms, and are relatively unstable in structure and performance [15], [16]. Therefore, development of a new SERS substrate to study the enhancement mechanism and expand the application fields of SERS technology is necessary. In recent years, numerous studies have shown that the metal–semiconductor interface has a unique optical property. Researchers explored the CT behavior induced in the system by adjusting the direct contact surface at the metal and semiconductors (TiO2, ZnO, CuO, and Cu2S, etc.) heterojunction [17], [18]. This provides a new strategy for the SERS mechanism in the metal–semiconductor composite system. For example, in the Cu/ZnO/PATP/Ag system, by adjusting the assembly method and energy level changes, reliable experimental conclusions can be drawn based on the difference in SERS spectra related to the material properties. When light irradiates the system, if the energy levels of the various materials match, then the CT resonance effect will be generated between the subsystems, thereby enhancing the Raman scattering intensity. With the development of metal–semiconductor research, three possible mechanisms have been employed to explain the CT process: (1) semiconductor → molecule → metal, (2) metal → semiconductor → molecule and (3) metal → molecule → semiconductor [19].
However, for wide-band-gap semiconductors, the low conduction band electron density and weak SPR make its use alone as a carrier to study the SERS effect difficult. Therefore, we combine such semiconductors with noble metals, integrate the EM and CM into a composite system, use the strong SPR of noble metals to induce CT, and adjust the enhancement mechanism by changing the geometric parameters of the semiconductor. This combined method can greatly enhance the SERS effect and contribute to our research. ZnSe has been widely used in the fields of optics and optoelectronic devices, but few SERS reports on it have been presented. We fabricated Ag-ZnSe composite SERS substrates via layer-by-layer sputtering technology and obtained a CT system by controlling the Ag SPR and ZnSe thickness. By discussing the CT contribution to SERS enhancement under the action of different excitation wavelengths, some basis for studying the SERS mechanism of wide-band-gap semiconductors is provided.
Section snippets
Materials
ZnSe and Ag targets were purchased from Beijing TIANRY Science & Technology Development Center, NH4OH (25%) and H2O2 (30%) were purchased from Sinopharm Chemical Reagent Co., Ltd., and PATP and sodium dodecyl sulfate were purchased from Sigma-Aldrich Chemical Co., Inc. Deionized water (18.25 MΩ cm−1) and ethanol were used throughout the work. All chemicals were purchased at the highest optional purity and used without further purification.
Preparation of Ag-ZnSe ordered arrays
Silicon wafers were handled with deionized water, NH4OH,
Results and discussion
SEM images were used to analyze the basic morphology of the assemblies. The sample shown in Fig. 1a was sputtered with pure Ag, and the sputtering power and time were 10 W and 2 min, respectively. Fig. 1b-e shows the layer-by-layer sputtered Ag and ZnSe; the sputtering power and time for Ag were 10 W and 2 min, respectively, and those for ZnSe were 60 W and 2, 5, 8, or 11 min. The sample shown in Fig. 1f was sputtered with pure ZnSe, and the sputtering power and time were 60 W and 11 min,
Conclusion
In this work, to analyze the possible SERS mechanism, the Ag-ZnSe-PATP system was designed, and its SERS spectra were measured with 514 and 633 nm laser excitations. The SPR effect of the metal was adjusted by controlling the geometric parameters of ZnSe. The CT process between the semiconductor and the probe molecule caused by the SPR effect of Ag was successfully analyzed, and the results indicated that with increasing ZnSe thickness, the highest SERS enhancement was dominated by the CT
CRediT authorship contribution statement
Qi Chu: Investigation, Writing - original draft. Bingbing Han: Investigation. Yang Jin: Validation. Shuang Guo: Formal analysis. Sila Jin: Formal analysis. Eungyeong Park: Investigation. Lei Chen: Writing - review & editing, Supervision. Young Mee Jung: Writing - review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This research was funded by the project of Jilin Development and Reform Commission (No. 2019C051-3). This work was also supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (No. NRF-2018R1A2A3074587 and No. NRF-2020K2A9A2A06036299).
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